This invention relates to an optical type displacement measuring apparatus which finds a displacement (distance) to an object to-be-measured by the use of a light source and a position sensing device (PSD). More particularly, it relates to an optical type displacement measuring apparatus which can eliminate a measurement error ascribable to the inclination of an object to-be-measured.
FIG. 5 is a block diagram of an optical type displacement measuring apparatus in a prior art described in, for example, an article "Laser Distance Sensing" in a magazine "O Plus E" (No. 82, September 1986), the apparatus being redrawn by the inventor in order to facilitate understanding.
Referring to the FIGURE,a head 1 for measuring a displacement includes a light source 11, a light source driver circuit 12 which drives the light source 11, a light projecting lens 13 by which a light beam L from the light source 11 is condensed on an object to-be-measured (not shown), a light receiving lens 14 which condenses light regularly reflected or scattered from the object to-be-measured, a PSD 15 by which the regularly reflected light or scattered light received is converted into two channels of current signals I.sub.1 and I.sub.2, and preamplifiers 16 and 17 which convert and amplify the respective current signals I.sub.1 and I.sub.2 into voltage signals V.sub.1 and V.sub.2.
An analog switch 2 turns the two channels of voltage signals V.sub.1 and V.sub.2 into one channel, whereupon a buffer amplifier 3 amplifies the voltage signals V.sub.1 and V.sub.2 of one channel.
An arithmetic circuit 4 includes a sample-and-hold circuit 41 in which the voltage signals V.sub.1 and V.sub.2 passed through the buffer amplifier 3 are held, an A/D (analog-to-digital) converter 42 by which the respective voltage signals V.sub.1 and V.sub.2 from the sample-and-hold circuit 41 are converted into digital signals D.sub.1 and D.sub.2, and a CPU 43 which delivers the displacement value P of the object to-be-measured on the basis of the digital signals D.sub.1 and D.sub.2.
A displacement value conversion table memory 5 connected to the CPU 43 stores therein the displacement value P which corresponds to a displacement signal S calculated on the basis of the digital signals D.sub.1 and D.sub.2.
Next, the operation of the prior-art, optical type displacement measuring apparatus shown in FIG. 5 will be described.
When a light source turning-on command is issued by the CPU 43, the light source driver circuit 2 drives the light source 1 made of an LD (laser diode) or an LED (light emitting diode). The light beam L emitted from the light source 1 is thrown on the object to-be-measured through the light projecting lens 13, and is regularly reflected (or is scattered) on the object to-be-measured. The regularly reflected light (or scattered light) is passed through the light receiving lens 14 into a light beam of small diameter, which is focused on one point of the PSD 15 at a position corresponding to the displacement (distance) of the object to-be-measured.
The PSD 15 converts the focused light beam into the two channels of current signals I.sub.1 and I.sub.2 corresponding to the position and intensity of the light beam, and delivers them from both the ends thereof. Here, letting I denote a current signal which corresponds to the total intensity of the received light beam, the following holds: EQU I=I.sub.1 +I.sub.2
The current signals I.sub.1 and I.sub.2 are respectively converted and amplified into the voltage signals V.sub.1 and V.sub.2 by the preamplifiers 16 and 17. Further, these voltage signals are brought into one channel in time-division fashion by the analog switch 2 and are amplified by the buffer amplifier 3, whereupon they are applied to the arithmetic circuit 4.
After the voltage signals V.sub.1 and V.sub.2 have been held in the sample-and-hold circuit 41, they are converted by the A/D converter 42 into the digital signals D.sub.1 and D.sub.2, which are fed into the CPU 43. On the basis of the digital signals D.sub.1 and D.sub.2, the CPU 43 calculates the displacement signal S (-1.ltoreq.S.ltoreq.1) which is given by: EQU S=(D.sub.1 -D.sub.2)/(D.sub.1 +D.sub.2) (1)
It obtains the displacement value P corresponding to the displacement signal S from the displacement value conversion table memory 5, and delivers this displacement value P. Alternatively, in a case where the displacement value conversion table memory is not employed, the displacement value P is obtained in such a way that a correctional calculation is performed by multiplying the displacement signal S by a coefficient.
However, assuming that the surface of the object to-be-measured 10 is not perpendicular to the light beam L but inclines an angle .theta. as illustrated in FIG. 6, a light spot which is formed on the surface of the object to-be-measured 10 by the light beam L has a certain degree of diameter, and hence, the regularly reflected light (or the scattered light) is disturbed delicately. Accordingly, an error is involved in the displacement signal S based on the current signals I.sub.1 and I.sub.2, to eventually incur an error in the delivered displacement value P. FIG. 7 is a graph of the relationship between the displacement value P and the error H as has been experimentally obtained. The graph indicates that, as the inclination angle .theta. and the displacement value P enlarge more, the error H becomes greater.
As stated above, the optical type displacement measuring apparatus of the prior art directly delivers the displacement value P based on the displacement signal S. Therefore, it has had the problem that, in the case where the surface of the object to-be-measured 10 inclines relative to the optical axis of the head 1, the current signals I.sub.1 and I.sub.2 from the PSD 15 change to contain errors, so the displacement value P of the object to-be-measured 10 changes apparently.